Association between subclinical hypothyroidism and vitamin D deficiency: Insights from a case-control study

Abstract

Subclinical hypothyroidism (SCH) is biochemically characterized by elevated thyrotropin (thyroid-stimulating hormone [TSH]) levels, while free thyroxine (FT4) levels remain normal. Given the high prevalence of vitamin D deficiency in Iran, investigating the association between vitamin D levels and SCH may improve treatment. A case-control study was conducted at the endocrinology clinic of Imam Khomeini Hospital, affiliated with Ahvaz Jundishapur University of Medical Sciences. A total of 106 subjects who met the inclusion criteria were selected and divided into 2 groups: 53 subjects with SCH and 53 healthy controls (HC). Serum levels of vitamin D, TSH, and FT4 were obtained from medical records. Other variables including age, weight, and height were also recorded. Statistical analyses were performed using the Statistical Package for the Social Sciences (SPSS) version 26. The mean age of the SCH and HC groups was 35.98 ± 12.9 years and 34.34 ± 13.49 years, respectively. In the SCH group, the mean serum vitamin D level was 23.99 ± 9.26 ng/mL, compared to 26.73 ± 9.42 in the HC group, with no statistically significant difference (P = .10). In both groups, no significant correlation was found between vitamin D levels and variables, such as sex, age, and body mass index. However, the TSH and FT4 levels in the SCH group were significantly higher than those in the control group. No significant difference in vitamin D deficiency was observed between SCH patients and HC.

1. Introduction

Subclinical hypothyroidism (SCH) occurs when thyroid hormone levels remain within the normal range, but thyrotropin (also known as thyroid-stimulating hormone [TSH]) levels are mildly elevated.^[1] Since most patients with SCH are asymptomatic, thyroid function tests are the only biochemical diagnostic tests. In iodine-sufficient populations, SCH affects up to 10%, primarily women and the elderly.^[1] The hypothalamic-pituitary-thyroid axis is regulated by a negative feedback loop that controls thyroid hormone levels.^[2] Even a slight decrease in thyroxine (T4) levels disrupts this balance, causing a nonlinear increase in TSH levels, which can lead to hypothyroidism.^[3] The individual ranges for TSH and T4 are often narrower than the population reference range, indicating that an individual’s TSH levels may exceed the upper limit of the reference range owing to genetic factors.^[4] In addition, thyroid inflammation or intrinsic thyroid disease in SCH can prevent thyroid hormone production from increasing in response to elevated serum TSH levels, leading to persistently elevated TSH levels.

Vitamin D (calciferol) is a fat-soluble vitamin that plays a key role in mineral ion homeostasis, particularly calcium homeostasis.^[5] It is naturally present in certain foods and is produced endogenously when ultraviolet B rays from sunlight trigger its synthesis in the skin.^[6] Globally, over a billion individuals suffer from vitamin D deficiency, often due to inadequate ultraviolet B exposure or insufficient dietary intake.^[7] Serum calcidiol (25(OH)D) levels are the most reliable measure of vitamin D status, with deficiency typically defined as a serum 25(OH)D concentration of ≤20 ng/mL.^[8] Beyond its role in calcium homeostasis, vitamin D acts similarly to steroid hormones, influencing gene transcription by binding to high-affinity receptors in the target cells. Recent studies have linked vitamin D to various extra skeletal functions, including immune regulation, cancer prevention, and the management of diseases such as type 1 diabetes, asthma, cardiovascular disease, and rheumatoid arthritis.^[9] Vitamin D has been proposed to boost immunological and anti-inflammatory responses by directly or indirectly inhibiting cytokine synthesis.^[10] Evidence also suggests a potential role of vitamin D in thyroid health. Calcitriol, the active form of vitamin D, exerts biological effects by binding to nuclear vitamin D receptors, which belong to the same receptor family as thyroid hormone and retinoid receptors.^[10] Since data on the association between vitamin D deficiency and SCH are limited, particularly in our region, this study aimed to evaluate serum vitamin D levels in patients with SCH compared to healthy controls (HC).

2. Methods

This observational case-control study was approved by the Medical Ethics Committee of the Ahvaz Jundishapur University of Medical Sciences (approval no. IR.AJUMS.HGOLESTAN.REC.1402.084). The medical records of patients who attended the Endocrinology Outpatient Clinic of Imam Khomeini Hospital, affiliated with Ahvaz Jundishapur University of Medical Sciences in the Khuzestan region of Iran, were reviewed. The study period spanned from October 2023 to January 2024. We calculated the sample size using G*power software, resulting in a total of 106 subjects: 53 HC and 53 newly diagnosed subclinical hypothyroid patients. The HC group were euthyroid, with normal thyroid profiles and no history of thyroid disorders. Subclinical hypothyroid patients had TSH levels between 5.0 and 10 mIU/L, with normal FT4 levels, indicating subclinical hypothyroid dysfunction. Subjects who had undergone thyroid surgery or had received radioactive iodine therapy, levothyroxine, or vitamin D supplementation were excluded. Pregnant or breastfeeding women were excluded from this study. Clinical data, including TSH, FT4, vitamin D, age, sex, height, and weight, were collected from the medical records. Body mass index (BMI) was calculated for each participates using the following formula: weight (kg) divided by height (m²). Vitamin D deficiency was defined as a serum vitamin D level ≤20 ng/mL.

2.1. Statistical analysis

Descriptive statistics were used to summarize categorical variables. The Kolmogorov–Smirnov test assessed the normality of the data. For non-normally distributed data, the Mann–Whitney U test was used to compare independent groups. The chi-square test (χ²) was used to evaluate categorical variables and compare proportions between the groups. The Spearman’s rank-order correlation coefficient was used to examine the relationships between continuous variables that did not follow a normal distribution. A significance level of 0.05 was considered in all tests. Data analysis was performed using the Statistical Package for the Social Sciences (SPSS) version 26.

3. Results

Table 1 presents the demographic characteristics of the 106 participants. The mean age in the SCH group was 35.98 ± 12.9 years, compared with 34.34 ± 13.49 years in the HC group. Female comprised 81% SCH group and of 68.3 % HC group. The mean vitamin D level in the SCH group was 23.99 ± 9.26 ng/mL, compared to 26.73 ± 9.42 ng/mL in the HC group. However, using the Mann–Whitney U test, the difference was not statistically significant (P = .10). Table 2 shows the distribution of subjects in both groups categorized by vitamin D deficiency severity and sex, with more women than men suffering from vitamin D deficiency in both groups. As expected, based on the definition of SCH, the TSH levels were significantly higher in the SCH group (P < .001). The FT4 levels were also higher in the SCH group (P = .002). In contrast, there was no significant difference in BMI between the SCH and HC groups (P = .12). Table 3 presents the results of the correlation analysis. Spearman’s test revealed no correlation between vitamin D and TSH levels (P = .10). In contrast, a weak but significant positive correlation was found between FT4 and vitamin D levels (ρ = .19, P = .04). This result suggests a slight association between FT4 and increased vitamin D levels. However, no significant correlations were observed between vitamin D levels and age or BMI (Fig. 1). The datasets generated and/or analyzed are available from the corresponding author upon reasonable request.

Table 1.

The demographic characteristics of SCH and HC group.

	SCH subjects (N = 53)	HCs (N = 53)	P value
Sex, female	43 (81)	36 (69)	.11*
Age, yr	35.98 ± 12.9	34.34 ± 13.49	.51**
TSH, mU/mL	6.84 ± 1.11	2.37 ± 1.14	<.001**
FT4, ng/mL	1.28 ± 0.22	1.16 ± 0.19	.002**
Vitamin D, ng/mL	23.99 ± 9.26	26.73 ± 9.42	.10**
BMI, kg/m2	25.61 ± 3.97	27.34 ± 6.18	.12**

Sex is presented as N (%). Other data are presented as the mean ± SD.

BMI = body mass index, FT4 = free thyroxine, HCs = healthy controls, N = number, SCH = subclinical hypothyroidism, SD = standard deviation, TSH = thyroid-stimulating hormone.

^*Mann–Whitney U test.

^**Chi-square test.

Table 2.

Individuals in both groups (SCH and HC) categorized by the severity of vitamin D deficiency and gender.

	Vitamin D deficiency	Vitamin D insufficiency	Normal vitamin D
SCH (N)	19	23	11
Female	18	19	7
Male	1	4	4
HC (N)	18	15	20
Female	13	13	10
Male	5	2	10

HCs = healthy controls, N = number, SCH = subclinical hypothyroidism.

Table 3.

Correlation between vitamin D and other parameters.

	ρ*	P value
TSH	−.15	.10
FT4	.19	.04
BMI	.007	.94
Age	.07	.44

BMI = body mass index, FT4 = free thyroxine, TSH = thyroid-stimulating hormone.

^*Spearman ρ correlation coefficient.

Figure 1.

Scatter plot showing the relationship between serum TSH and vitamin D levels in patients with subclinical hypothyroidism and healthy controls. No significant correlation was observed (P = .10). TSH = thyroid-stimulating hormone.

4. Discussion

Vitamin D is known for its role in calcium homeostasis, but its biological effects extend beyond skeletal functions.^[11] New evidence suggests a complex connection between vitamin D and thyroid physiology, suggesting that vitamin D may positively influence thyroid function.^[12] In addition, thyroid follicular cells, including those involved in papillary thyroid carcinoma, express the vitamin D receptors and the enzyme 1-alpha hydroxylase, indicating a potential for local calcitriol (the active form of vitamin D) production within the thyroid.^[13] Despite these findings, our study found no statistically significant difference in vitamin D levels between the SCH and HC groups (P = .10), suggesting that the relationship between vitamin D and thyroid function may be more complex than previously thought. However, the mean serum vitamin D level was lower in the SCH group (23.99 ng/mL) than in the HC group (26.73 ng/mL). In addition, both groups had a high prevalence of vitamin D deficiency, a common concern in the Iranian population. According to our data presented in Table 2, 79.2% of SCH patients and 62.2% of HC had vitamin D levels below 30 ng/mL, which is consistent with previous meta-analyses.^[14] These results are in contrast to some studies that reported lower vitamin D levels in patients with hypothyroidism,^[15] while other studies align with our results and show no significant association between vitamin D and thyroid dysfunction.^[16,17] Interestingly, Aljohani et al showed significantly higher mean vitamin D levels in SCH patients (P < .001).^[18] Differences in dietary intake, region, and other comorbidities that may not be documented are some unmeasured confounders that could explain these differences. Women are generally more susceptible to thyroid disorders, including SCH.^[19] In this study, 81% of the SCH group and 69% of the HC group were female. On the other hand, according to the distribution of enrolled subjects by severity of vitamin D deficiency, more women than men suffer from vitamin D deficiency. This observation suggests that although gender may play a role in thyroid disease, it did not have a significant impact on vitamin D levels in our study population. In addition, a significant positive correlation was found between FT4 and vitamin D levels (P = .04). This suggests a possible association between higher FT4 levels and increased vitamin D. As expected, SCH group had significantly higher TSH and FT4 levels than HC group, confirming the diagnostic criteria for SCH. However, what is notable is the lack of a significant difference in BMI between the 2 groups (P = .12). However, obesity is often associated with both hypothyroidism and vitamin D deficiency.^[20,21]

Recent studies have examined vitamin D supplementation and its impact on TSH levels in SCH patients. Safari et al found that vitamin D supplementation not only significantly reduced TSH levels but also reduced fat-free mass in women diagnosed with SCH.^[22] Pezeshki et al similarly observed a significant reduction in TSH levels in SCH patients with vitamin D deficiency after administration of vitamin D replacement.^[23] While our study did not find a significant correlation between TSH and vitamin D levels, these results highlight the potential benefit of vitamin D as adjunctive therapy in the treatment of SCH. However, further research is required to fully clarify the connection and the mechanism. Our results, showing a weak but nonsignificant negative correlation between TSH and vitamin D levels, emphasize the complexity of this relationship. Factors such as the severity of SCH or the duration of vitamin D deficiency may influence these outcomes.

5. Limitations and future directions

This study has some limitations. The small sample size may have limited the ability to detect significant differences. In addition, we did not consider confounding factors such as seasonal variations that may affect vitamin D status. Further studies with larger samples are needed to better understand the association between vitamin D and SCH, particularly in patients with concurrent vitamin D deficiency. Investigating the potential role of vitamin D supplementation in SCH treatment may also provide valuable insights.

6. Conclusion

In summary, our results suggest that vitamin D deficiency is common in both SCH and HC group and that there is no significant association between vitamin D levels and SCH in the adult Iranian population. Further research is needed to elucidate the complex interplay between vitamin D, thyroid function, and other factors in SCH.

Author contributions

Conceptualization: Amiri Fatemeh, Zangeneh-Yousefabadi Elham.

Data curation: Amiri Fatemeh, Chitzan-Zadeh Kosar.

Formal analysis: Moradi Maryam.

Investigation: Chitzan-Zadeh Kosar.

Methodology: Amiri Fatemeh, Zangeneh-Yousefabadi Elham.

Project administration: Chitzan-Zadeh Kosar.

Resources: Amiri Fatemeh, Chitzan-Zadeh Kosar.

Software: Chitzan-Zadeh Kosar.

Supervision: Amiri Fatemeh, Zangeneh-Yousefabadi Elham, Rashidi Homeira.

Validation: Amiri Fatemeh, Zangeneh-Yousefabadi Elham.

Visualization: Chitzan-Zadeh Kosar.

Writing – original draft: Chitzan-Zadeh Kosar.

Writing – review & editing: Amiri Fatemeh, Zangeneh-Yousefabadi Elham, Chitzan-Zadeh Kosar, Rashidi Homeira.